The present invention relates to a thermal turbomachine with a housing, a combustion chamber, and an ignition device, and a process for igniting the thermal turbomachine.
In order to start a continuous chemical reaction, the combustion chambers of furnaces and gas turbine plants are in general ignited by means of so-called ignition aids. In particular, electrical ignition or glow plugs are known for this purpose; however, a gas flame (ignition torch) is also frequently used.
Auxiliary burners are mostly used first, which then light the main burner. In gas turbines the main fuel is also partially used for the ignition burner, i.e., according to the case of application, fuel oil, kerosene or natural gas. Easily storable auxiliary gases such as propane, for example, are of course also occasionally applied.
Successful ignition by means of the ignition aids depends on numerous factors. Among these are the inlet conditions of the combustion air, such as pressure, temperature and velocity, the ignition fuel used and its conditions, the geometry and makeup of the combustion chamber, and finally also the composition of the combustion air.
The ignition of combustion chambers in thermal turbomachines, such as, for, example, in gas turbines, by means of an ignition plug or glow plug, is described in, among others, A. Lefebvre in xe2x80x9cGas Turbine Combustionxe2x80x9d (Arthur H. Lefebvre: Gas Turbine Combustion, pages 222-225 and pages 232-233, Hemisphere Publishing Corporation, 1983).
As a matter of course, a condition for the function of the ignition device is that conditions are present at the igniter which make it possible for a stable and sufficiently powerful ignition flame to arise. To be mentioned here in particular are the adjustment of the fuel-air ratio and also an appropriate slow speed in the region of the igniter.
These conditions can often be fulfilled in the combustion chambers of gas turbines only by the use of special ignition burners. The igniter is therefore frequently installed in a small ignition space, so that only a small volume has to be ignited by the initial ignition. A stable ignition flame arises in succession in a flame tube situated downstream of this ignition space.
Such an ignition system for gas turbines is known from the document EP-A1-992 661. Other igniters are known from, e.g., U.S. Pat. Nos. 2,967,244, 3,404,939, 5,368,474, or from the international publication WO 96/39578. A burner with an ignition device for gaseous fuels and oxygen is also described in the document DD 241 457. Furthermore, a process for igniting the main flame of a combustion chamber is known from the document DE-A1 4 320 429.
Under the conditions which are present in the combustors of modern gas turbines, the ignition system requires a high maintenance cost to preserve its reliable functioning over a long period. In all the above-mentioned documents, this in particular takes place by the arrangement of the igniter in the immediate neighborhood of the combustion chamber and of the high thermal loading associated therewith. Since the ignition takes place in the interior of the thermal turbomachine the high voltage has to be conducted there through the air channel of the compressor exit air. High temperatures likewise prevail here, so that a corresponding ceramic insulation has to be provided for the current-carrying parts.
The life of the igniter is thus limited by the high stress on the materials used, the thermal gradients and transients which arise, and also by vibrations.
Known glow cylinders used in automobile construction and not requiring such highly efficient insulation because of their low electrical voltage cannot be used, because they change over time during the operation of the machine, due to the high combustion chamber temperatures.
In view of the above-discussed disadvantages encountered in the prior art, the invention increases the total life of the igniter by a reduced thermal loading, in a thermal turbomachine with a housing, a combustion chamber, and an ignition device. The invention also provides a process for the ignition of the thermal turbomachine, by means of which the life of the igniter is increased.
The ignition device is arranged outside the housing of the thermal turbomachine and is connected to the combustion chamber by an ignition tube. The process for the ignition of the thermal turbomachine according to the invention includes a fuel/air mixture being introduced into the ignition space and the ignition tube, the fuel/air mixture being ignited in the ignition space outside the housing of the thermal turbomachine, and the ignition flame is led through the ignition tube into the combustion chamber.
The fuel/air mixture is mixed in the ignition space and the ignition tube before the ignition. By a suitable choice of the inflow speed of the two media, air/fuelxe2x80x94higher than the flame speedxe2x80x94the ignited flame then runs through the ignition tube toward the combustion chamber. The flame is stabilized there due to the widening of cross section. The ignition of the main burner then takes place by means of the ignition flame burning in the combustion chamber. By this arrangement and by this process, the thermal load on the igniter, the electrical parts and all the ducts, is reduced in all, since the igniter is outside the housing and thus not exposed by the combustion chamber to high temperatures. Furthermore, the ignition elements are not exposed to the high thermal radiation of the flame, nor to hot gases of the combustion chamber sweeping past them.
In an advantageous embodiment, the ignition tube can be directly connected to the main combustion chamber. For better ignition, the ignition point can be equipped with a widening of cross section toward the combustion chamber. It is also conceivable to embody the transition to the ignition tube as a nozzle.
Another advantageous embodiment consists of arranging the cross section widening for stabilizing the ignition flame, not directly on the combustion chamber wall, but stepped in the form of a flame tube between the combustion chamber and the ignition tube. This can be connected such that the flame tube is connected, via at least a further duct to each of the fuel supply and the air supply, so that the ignition flame is conventionally supplied with fuel and combustion air within the gas turbine. In particular, this has the advantage that the fuel/air amounts of the igniting torch can be adjusted independently of the amount of fuel of the ignition tube. The ignition of the ignition burner of course takes place outside the machine, through the ignition tube which is filled with combustible mixture. The ignition flame runs through the ignition tube toward the combustion chamber and there ignites the separately supplied fuel/air mixture. The filling of the ignition tube correspondingly takes place only once for the production of the ignition flame. After this, fuel and/or air can be supplied to the combustion chamber by means of the ignition tube.
The igniter itself has to ignite only a small volume of mixture. Thus the minimum energy to be applied for a reliable initial ignition is reduced. The ignition flame thus arising passes from the ignition space through the ignition tube into the flame tube, or directly into the combustion chamber, where it ignites the fuel-air mixture present there, so that finally a main ignition flame arises for the whole combustion chamber.
In an embodiment of an ignition device according to the invention, the flow of fuel and air around the igniter is effectively decoupled from parameters such as the pressure in the combustion space and the initial pressure of air and fuel. Good ignition conditions can thus be ensured directly at the igniter over a wide operating range of the ignition device.
A mixing chamber into which the fuel supply and the air supply open can advantageously be arranged before the ignition space, so that an improved mixing and a reliable ignition can take place.
The igniter can be embodied as a glow igniter or as a spark igniter. Aging processes due to thermal action no longer play any part.
For reliable ignition, the ignition space and/or the ignition tube can be catalytically coated, and also suitable ignition aids (oxidizers, easily ignited reaction materials, etc.) can be used. For the same purpose, the ignition flame can be stabilized by swirl at the outlet of the ignition tube.